1. Field of the Invention
This invention relates to methods for increasing resistance of plants to plant pathogens. More specifically, this invention relates to the surprising discovery that the application to plants of one or more reactive oxygen species and of one or more plant systemic inducers, either simultaneously or within a short time of each other, results in an increase in the level of pathogenesis-related proteins and of systemic acquired resistance in the plants over the effect of either one alone.
2. Background
Commercial cultivation of plants is a major part of the economy, encompassing not only crops grown for human food and animal feed, but also those, like cotton, grown for fiber, and others, such as flowers, grown for beauty. The importance of plants to people and to the economy can hardly be overstated. Plants are, however, also subject to constant attack by insects, fungi, bacteria, viruses, nematodes, and other pathogens. When pathogens find susceptible plants, these attacks can result in the loss of yield and quality, and may result in the loss of entire crops. These losses result in substantial economic harm to the growers and, in some areas of the world, contribute to famine.
Except for those farmers who practice organic farming, most attempts to control pathogens involve the use of pesticides, such as fungicides and insecticides. Many pesticides, however, have been withdrawn from the market because they have undesirable environmental impacts, and many currently on the market are being scrutinized for their environmental impact and may be withdrawn in the future. In addition, few, if any, pesticides are effective against the full range of pests which may attack a given crop from sowing to harvest to post-harvest storage. Thus, a number of different pesticides with different target organisms may need to be applied. Each one must be applied at the correct time in the growth of the plants to provide effective control of the target organism, each has its own requirements for handling and application, and each may require different, specialized equipment. Moreover, many pesticides are toxic or have toxic residues, and their use is therefore often restricted to certain windows of time before harvest, after which they cannot be used because of the potential danger to the consumer. During this window, the crop may be essentially unprotected, or yet another agent, safer for use close to harvesting, may be needed. The use of traditional chemical agents therefore requires complicated planning, careful timing, and considerable effort.
While pesticides form the bulk of attempts by farmers to protect plants from pathogenic attack, not all protection of plants against pathogens comes from the application of pesticides. For decades, it has been known that plants also have a wide variety of structural and biochemical defenses against attack by pathogens. See, e.g., Agrios, G., Plant Pathology, Academic Press, San Diego Calif. (3rd ed., 1988).
One of the biochemical defenses produced by plants in response to attack is induced resistance, in which plants which have been inoculated with biological agents or pretreated with various chemicals develop nonspecific resistance not only to the initial agent itself, but also to a variety of pathogenic agents, such as viruses, fungi, bacteria, and some insects. Induced resistance usually commences in the area around the initial inoculation, but over the course of a few days, may spread to portions of the plant not inoculated, a phenomenon known as systemic induced resistance, or as systemic acquired resistance (“SAR”).
A number of compounds, such as salicylic acid, can induce resistance. See, e.g., Klessig, D. and Malamy, J., Plant Mol. Biol., 26:1439–1458 (1994); Raskin, I., Annu. Rev. Plant Physiol. Plant Mol. Biol. 43:439–463 (1992). They can be used to induce local resistance, by injection or spraying, or to induce SAR when absorbed, for example, through the roots. See generally, Agrios, supra, at Chapters 5 and 9. SAR develops some 7 days or more after exposure to the inoculant or chemical agent, and usually lasts for some 3 to 5 weeks. Id.
Because SAR protects plants against many different pests, increasing SAR in crops could potentially decrease or even eliminate the need to apply toxic pesticides. Further, since SAR protects against a multitude of pathogens, inducing SAR can eliminate the need for a number of separate agents which would otherwise be necessary to protect a crop, or reduce the amount of the separate agents which would otherwise be required. And, because the induction of SAR can essentially be performed by repetitive action, use of this technique would demand far less effort for the farmer than the currently required regimen of applying multiple agents, each with their own directions for handling, timing, amounts, concentrations, methods of application, and possible adverse interactions.
One of the world's largest pharmaceutical companies has made an effort to develop the use of systemic inducers to protect crops in the field. To this end, it is bringing to market a systemic inducer, benzothiadiazole, under the trade name Actigard.™ But, the manufacturer now recommends that Actigard™ be used in combination with conventional chemical agents in providing protection to crops. Thus, even a systemic inducer specifically selected, developed and tested for protection of crops has not eliminated the need for conventional pesticides even during the time the systemic inducer is being applied.
U.S. Pat. No. 5,607,856, teaches compositions and methods for sterilizing soil using oxygen radicals. The method involves contacting the soil with a solution of an activated oxygen species, a water-soluble phenolic complex extracted from a material such as humic material, a divalent cation, and a cation redox reducing agent.
What is needed in the art is a means of protecting a variety of crops, flowers, decorative and other plants in the field from pathogens more effectively, at lower cost, and with less effort than by the use of pesticides and other traditional chemical agents. Moreover, what is needed is a means of providing this protection with lower and less lasting damage to the environment than caused by such conventional agents. What is further needed is a means of increasing the protection of crops from pathogens to levels above the levels obtainable by the use of systemic inducers alone, to more crops than can be protected by the use of systemic inducers alone, and against a wider range of pathogens. The present invention addresses these and other needs.